Method of identifying radioactive compounds



June 3, 1952 A, 1 DEMPSTER 2,599,166

METHOD OF IDENTIFYING RADOAC'IIVE COMPOUNDS Filed Aug. 17, 1945 fifa Wiley.-

v'l'ait'ented June 3, 1952 'fzisssglee mdiabrics" chemical lmestrictas: e

wrm-increment .of trici. "esetin "enti 'separately with the accompanyingdrawingwiereinz Fig. 1. is a schematic diagram illustratingfa masssctrograph for separating the-c``stitu ents' fia radiactive'mitureacordaiceftvth "the" different 'masses tlre;

Figf 2 is "an 'ipandd'viewft`a"collectr` plate or film afterL-such--plfateknrilnfhas been cut-up or Hsubdivided into sections carryingdiferent masses Aorl groups `of masses-f in-accordance-'fwith the principles of the present invention; and

Fig. 3 is a longitudinal cross sectional view, somewhat schematic, of a modification of the method for lseparately measuring the individual radioactivity of each of the constituents of different masses deposited cn the collector plate or nlm, shown in Fig. 1.

Referring more particularly to Fig. l, numeral l denotes an evacuated tube or envelope having therein a positive ion source which may be in the form of a heater filament 2 coated with a mixture, compound or element whose radioactivmelma",1er-'awt nigh riesame pushed jbyf'grouriiig disk at One-drfrr'orelncyn radica-etivefmaterialsl-hav; ing a known half life may be incorporated in the ion source to serve as reference indicators for determining the relative position of the unknown radioactive material or materials. Preferably, the quantity of such indicators is relatively large compared with the expected quantity of the materials it is desired to identify. By the position at which one or more of the indicators are deposited on the plate or lm 1, the position at which the deposits of the unknowns occur indicates the mass thereof, and the radioactivity indicates the relative abundor more reference points is valuable in obtain-` ing rapid indication of the mass numbers of the elements or isotopes for which identification is desired.

After the constituents of the ion source are separately .deposited on the film or plate 1 in accordance with their different masses, the collector plate or film is removed from the tube i and subdivided into sections representing different groups of masses, or better still, individual masses, particularly when the masses are of special interest. Such subdivided plate or film is shown in Fig. 2. A simple method for doing this is to use an ordinary gelatinous fllm as the collector, since it provides a surface upon which radioactive products readily adhere, and then to cut up the lm into separate parts representing different masses, that is. mass numbers. as may be done by suitable calculation involving the -geometry of the mass spectrograph tube as well .as a knowledge of the value of eld strength .and accelerating voltage used. Of course,vinstead of Vmaking the nlm or plate 1 in one piece to start out with, separate segments, such as shown in Fig. 2, may be provided before deposition which would eliminate the necessity. of cutting-up or otherwise subdividing the film after .deposition of the radioactive material thereon.

After the radioactive constituents are deposit ed and the lm has been subdivided, as shown in Fig. 2, each of the subdivisions is separately placed in a radiation detector, such as, for example, a Geiger-Mller tube (not shown), of any well-known type, and the radioactivity of each l subdivision is determined. The counting rate, which is indicative of the intensity of the particular constituent of mass, is noted, and the radioactive decay curves may be individually vplotted to show the various half-lives of different duration of lthe components of each incremental portion of the film. That is to say, the decay curve for a particular mass number may be plotted and then analyzed for its component parts, and therefore, the different elements on a given mass number of a radioactive decay Achain may be ascertained. By noting the. amplitude of the initial heights of each individual decay curve, it is possible to determine the rela- ',tive magnitude or relative abundance ratios of the various constituents.

An alternate way of measuring the radioactivity of each of the incremental portions of the collectorlm is to scan the collector film with a Geiger-Mller tube, as shown in Fig. 3.

Referring more particularly to Fig. 3, numeral 8 denotes a shielded container such as, for example, a cylindrical shield of lead, which includes therein a Geiger-Mller tube 9 of well known construction, or other suitablerradiation measuring device. A plate I0, having a slit Il, is disposed below the radiation measuring tube 9. The collector plate or film 1 is rigidly placed on top of a rack l2 which rack extends into the container 8, and is moved longitudinally by means of a manually rotatable pinion I3. Inasmuch as rack l2 is suitably calibrated, the degree of inward movement thereof may be observed by noting the position of the rack with respect to a stationary index, such as the side wall of the container or chamber 8. By suitable calibration of rack i2, it may lbe determined exactly what portion or mass number on plate 1 is being exposed to the radiation measuring device 9 through slit Il. In this manner, therefore, plate 'I may be scanned for its radioactive components, or constituents of different mass number, and the radioactive mass spectrum may be determined. In some instances, it may be desirable merely to find the particular mass number of greatest radioactivity to identify different elements of the same mass number in a particular radioactive decay chain.

From the above description it will be geen that there is provided an efiicient method and apparatus -for effectively separating the radioactive components or masses from the inert or nonradioactive components of a sample containing radioactive material. Hence, it is possible to plot a simple spectrum which is restricted solely to the radioactive components of the sample under test forming the ion source.

It will be apparent that modifications will be readily suggested to those skilled in the art after having had the benefit of the teachings of the p-resent invention; hence, the invention should not be restricted except insofar as set forth in the following claim.Y

What is claimed is:

The method of identifying the radioactive components of a composition comprising adding to the composition known isotopes having known half-lives, ionizing the combination of the com position and the known isotopes, separating the ions into groups having Vdifferent mass-tocharge ratios, collecting each group individually upon a non-radioactive adhering collector, and measuring at known time intervals the radioactivity of the material deposited by each group upon the collectorrplate, whereby'the chemical elements that constitute each group of common mass number mai7 be determined by the characteristics of theradioactive decay of the group.

VARTHUR. J. DEMPSTER.

REFERENCES CITED The following references are of record in th le of this patent:

UNITED STATES PATENTS Number Name Date 2,221,467 Bieakney Nov. 12, 1940 2,316,361 Piety Apr. 13, 1943 2,355,658 Lawlor Aug. 15, 1944 OTHER REFERENCES Physical Review, Apr. 15, 1940, vol. 57, p. 748. Physical Review, Mar. 15, 1940, vol. 57, p. 546. 

